Darnestown, Sept. 23, 1861.

--A tragical occurrence was witnessed near this village yesterday in the Pennsylvania Forty sixth, Col. Knipe, on their way to join Gen. Banks's column. In the regiment is a company composed principally of Irishmen, who have given the commander much trouble. When near Muddy Run two of the men got engaged in a fight, and one of them, named [John] Lanahan, of Company I, a very bad character, was, by order of Major Arnold C. Lewis, tied to the rear of a baggage wagon. After proceeding about a mile it was reported to Major Lewis that Lanahan was at large with a loaded gun in his hand. The Major, accompanied by Assistant Surgeon W. Charles Rogers, rode towards Lanahan, and ordered him to give up the gun. Lanahan refused; and while the Major (Lewis) was in the act of getting off his horse to enforce his order, Lanahan retreated a few steps, took deliberate aim, and shot the Major in the back with a ball and three buck-shot. The latter fell and expired in three minutes without uttering a word. Lanahan was secured, and the body of the unfortunate officer was placed in a wagon and guarded to the camp. Lanalian is now in the hands of the Provost Marshal, and will be tried by a drum- head court probably to-day. The murderer is a resident of Scranton, Pennsylvania. Major Lewis was thirty-two years of age, was married about a year ago, and leaves a widow but no children. He served in the Mexican war, and on the breaking out of the present rebellion was editing the Catasauqua (Lehigh county) Herald, which occupation he resigned to enter the army. He was dearly beloved by the whole regiment, and his death hangs like a pall upon the spirits of his associates.